Refrigeration system and method



March 6, 1956 LATHAM, JR 2,737,032

REFRIGERATION SYSTEM AND METHOD Filed Feb. 4, 1952 Fig. I I8 325|/'4G66K /l6 OA 96K COOLER PURI' Y'ILVL'A/A I E l0 HEAT EXCHANGER' HEREXPANS'ON 5 :l COMPRESSOR 32|K 62K -figgfiifi/hi 960K DURI- /38 HEREXPANSION LOAD I 59 32 I COMPRESSOR 32 K I, 920K (,I\:I- .,\Y,, 620KFig. 3 300 Fig. 2

EXPANSION K LOAD\ ENGINE EXPANSION LOAD ENGINE O J HEAT EXCHANGER L JFg- HEAT EXCHANGER ADSORPTIVE CAPCITY, log X=AkVT INVENTOR. 4 ALLENLATHAM, JR.

j ozag' .v-W T. 1;: 7;

ATTORNEYS United States Patent REFRIGERATION SYSTEM AND METHOD AllenLatham, Jr., Boston, Mass., assignor to Arthur D.

Little, Inc., Cambridge, Mass., a corporation of MassachusettsApplication February 4, 1952, Serial No. 269,721

6 Claims. (Cl. 62-11755) The present invention relates to refrigerationsystems and is directed particularly to low temperature refrigerationsystems employing the expansion of compressed refrigerant gases.

Where extremely low temperatures are to be produced by mechanicalrefrigeration, it is generally desirable to employ in the refrigerationcycle one or more gas expansion steps wherein external work is performedby the expanding gas. Such expansions are generally carried out inexpansion engines which include a member movable by the expansion ofcompressed gas in a closed chamber, and means operatively connected withthe movable member for transmitting work from the engine to anappropriate load. The extremely low temperatures at which these enginesare run precludes the use in them of lubricants. ,Accordingly, when thecompressed gas is expanded in such an engine, it is extremely importantthat itlbe as free as possible from contaminating materials, especiallysolids and materials condensable to solids. In refrigeration systemswherein the gas is expanded in a throttle valve, it is also importantthat the expanding gas be as free as possible from such impurities andthe present invention may be used in systems wherein expansion isperformed in a throttle valve. To remove impurities from the gas, it isthe common practice to pass the gas through a purifier prior toexpansion. In the purifier the gas is passed in contact with the surfaceof an adsorbent. It is well known that the adsorptive capacity ofadsorbents such as charcoal which are used to purify gases increases asthe temperature of the gas decreases. Moreover, as the temperature ofthe gas decreases, not only are some impurities adsorbed to a greaterdegree, but other impurities which can be adsorbed only slightly athigher temperatures are readily removed. In some cases the condensationof some impurities to the solid form may be effected if the temperaturein the purifier is sufliciently low, so that they may be filtered outmechanically.

It is a primary object of this invention to enhance the efiiciency ofthe purification of the refrigerant gas prior to the expansion stage. Afurther object is to cool the refrigerant gas prior to its expansion toa temperature only slightly higher than the temperature attained duringexpansion. This is desirable because it provides a maximum assuranceagainst the condensation of contaminating materials during the expansionand accompanying drop of temperature. Other objects will become apparentfrom the following disclosure.

In general, the objects of this invention are realized by passing thecooled, compressed refrigerant gas into the load (the source of heat tobe removed by refrigeration) before expanding it, then expanding the gasand passing the expanded cold gas into countercurrent out-ofcontact heatexchange relation with the compressed gas prior to its passage into theload. Thus, in effect, the

load is cooled by a secondary refrigerant, the compressed gas, which hasfirst been cooled by the primary refrigerant, the expanded gas. Bycooling the gas to r'efrigeration temperature, then passing it throughthe load before expanding it, the temperature of the gas is first raisedduring its passage through the load, then cooled by expansion to atemperature only low enough to be effective to cool the compressed gasto refrigeration temperature in a heat exchanger. A purifier may then besituated in the system between the heat exchanger and the load and itwill operate at a temperature very near the low temperature produced bythe expansion to provide maximum purification.

By comparison, a conventional system wherein the gas is expanded priorto its passage through the load, requires a purifier between the heatexchanger and the expansion engine, at a point in the system where thetemperature is above the entire temperature range of the expanding gas.

The present invention will be better understood from the followingdetailed description of a preferred embodiment thereof and a comparisonof the system of the present invention with a conventional refrigerationsystem. Reference is made to the drawings in which:

Figure 1 is a diagrammatic drawing of a refrigeration system embodyingthe present invention,

Figure 2 is a diagrammatic drawing of a conventional refrigerationsystem,

Figure 3 is a graph showing the temperature variations through thesystems shown in Figs. 1 and 2, and

Figure 4 is a graph showing the variation of the adsorptive capacity ofa typical adsorbent with the temperature.

Referring now to Figure 1, the system of the present invention includesa compressor 10 which discharges compressed gas into a conduit 12. Acooler 13 surrounds the conduit 12 to remove the heat of compressionfrom the compressed gas. The conduit 12 leads into one channel 14a of adouble channel heat exchanger 14 which in turn leads into a purifier 16.The purifier 16 communicates with the refigerating load 18 (the sourceof heat to be removed by refrigeration) and the load 18 communicateswith the inlet side of the expansion engine 20, which may be of eitherthe turbine or reciprocating piston type. The outlet side of theexpansion engine 20 communicates with the other channel 14b of the heatexchanger 14 which in turn, leads back to the low pressure side of thecompressor 10.

The heat exchanger is of conventional design and may consist of twoseparate channels thermally bonded together so that heat may be readilytransferred from a relatively warm fluid in one channel to a relativelycold fluid in the other channel. The purifier 16 is also of conventionaldesign and may consist of a container containing an adsorbent such ascharcoal or silica gel and means for passing the compressed gas incontact with the adsorbent.

In the operation of this embodiment of the present invention, arefrigerant gas such as helium is compressed in the compressor 10 andthen partially cooled in the cooler 13. In a typical operation, the gasis compressed to 230 p. s. i. a. and then cooled to 325 K. The gas isthen passed through the heat exchanger 14 and there brought into heatexchange relation with the expanded gas from the expansion engine, andcooled to refrigeration temperature of 66 K. The cold gas then passesthrough the purifier 16 and thereafter through the load 18 wherein itstemperature is raised to 96 K. by the heat removed from the load. Thegas then enters the expansion engine 20 and is expanded with theperformance of external work to a pressure of 57.5 p. s. i. a. to reduceits temperature to 62 K. The expanded gas then passes through the heatexchanger in a direction countercurrent to that of the compressed gasthereby to cool the compressed gas while becoming heated to 321 K.

The warm, expanded gas then returns to the compressor where it isrecompressed and returned to the refrigeration cycle.

To emphasize the advantages of the system provided by the presentinvention, a description ofa corresponding conventional refrigerationsystem will now be given. Such a system is shown in Fig. 2. It consistsof a compressor 30 which discharges into a conduit 32 which may besurrounded by a cooler 33. The conduit 32 leads into one channel 34a ofa heat exchanger 34. This channel of the heat exchanger 34 thencommunicates with a purifier 36 which in turn communicates with theintake side of an expansion engine 38. The outlet of the expansionengine 38 leads through the load 40 which in turn, communicates with theother channel 34b of the heat exchanger 34. This channel of the heatexchanger 34 then leads to the low pressure intake of the compressor 30.

In a comparable operation of this system, the gas is compressed to 230p. s. i. a. in the compressor 30 and then cooled to 325 K. in the cooler33. In the heat exchanger 34, the compressed gas is cooled to only 96 K.and passes at this temperature through the purifier. The gas is thenexpanded in the expansion engine 38 to a pressure of 57.5 p. s. i. a.and a temperature of 62 K. The cold, expanded gas then passes throughthe load 40 wherein its temperature is raised to 92 K. The gasthereafter passes back through the heat exchanger wherein it cools thecompressed gas and is warmed to 321 K., and then returns to thecompressor 30 to be rc-compressed and returned to the cycle. The systemof Fig. 2 is in general similar to that described in the Collins PatentNo. 2,458,894, and the various parts shown in both Figs. 1 andv 2 may beof the construction of corresponding parts of the Collinspatent.

A comparsion of the refrigeration system of the present invention with aconventional refrigeration system is shown graphically, in Fig. 3 whichshows the temperature of the gas at the various stages. in the cycles ofthe two systems. In the conventional system, which is represented by theline marked as Fig. 2, the gas is compressed and cooled to a temperaturefrom which is may be expanded to the desired refrigeration temperature.It is at this relatively high temperature that the gas is passed throughthe purifier. During the expansion stage which follows, the temperatureof the, gas is lowered further with the result that impurities whichwere not removed by the purifier may condense in the engine and cause itto foul. The danger of fouling is minimized in the system of the presentinvention, which is represented by the line marked Fig. l of the graphof Fig. 3. In this sys tern, the gas is passed through the purifier at atemperature appreciably lower than the temperature at which it entersthe expansion engine. This minimizes the danger that the cooling of the;gas which results from its expansion, will result in the condensation ofimpurities which were not removed by the purifier.

The advantages afforded in the system of this invention by passing gasthrough the purifier at a temperature near the low point of theexpansion, includes not only the maximum removal of condensablematerial, but also the maximum removal of all impurities which may beadsorbed. This latter effect is attained because the adsorptive capacityof typical adsorbents such as charcoal or silica gel, is greatlyenhanced by decreases in temperature. The adsorptive capacity ofcharcoal as, a function of temperature may be represented by theformula:

where X represents theadsorptive capacity in grams adsorbed per. gram ofcharcoal, A represents. a constant, K represents a constant, and Trepresents the temperature in absoluteunits.

A plot of. thisformula is'shown in Fig. 4. .Itwill be seen from. this.plot, that at the extremely'low temperatures encountered inrefrigeration systems of the type with which this invention isconcerned, a substantial increase in the adsorptive capacity resultswith even relatively minor decreases in the temperature of the gas.

The foregoing description is presented as illustrative of therefrigeration system of this invention. The temperatures and pressuresdescribed with respect to this system, represent typical conditionsencountered in systems of this type, and they should not be construed inany limiting sense. In this respect, it will be understood that theexpansion ratio may be varied considerably as may be the rate that heatis supplied by the load, and that, accordingly, the temperaturesthroughout the system may be varied within wide limits. Also material tothe conditions that exist in this system, is the efficiency of the heatexchanger, which is desirably constructed to provide a maximum of heatinterchange, so that a minimum temperature differential between the twostreams may be maintained. Accordingly, it is contemplated that manymodifications of the system described above will be apparent to thoseskilled in the art, and that such modifications maybe made withoutdeparting from the scope of this invention.

Typical applications of the system of the present invention describedabove include the rectification of air to produce liquid oxygen whereinthe load consists of a nitrogen condenser for the production of refluxsuch as described in Patent No. 2,458,894. This system may also be usedto cool storage vessels for liquefied gases wherein the load consists ofa cooling element in thermal contact with the stored liquefied gases. Ingeneral, the system of thisinvention may be used for the production oflow temperatures between the boiling points at atmospheric pressures ofhelium and oxygen, or higher if desired.

Having thus disclosed my invention, and described in detail a preferredembodiment thereof, I desire to claim and secure by Letters Patent:

1. A low. temperature compressed gas refrigeration system comprising aheat exchanger containing a compressed gas channel for conducting acompressed gas through the exchanger and an expanded gas channelthermally bonded to the compressed gas channel for conducting anexpanded gas through the exchanger, a refrigeration load communicatingwith the compressed gas channel, means for expanding the compressed gasthrough a temperature decrease larger than the temperature increasethrough the load, means for conveying said compressed gas after its,passage through the load to the expansion means, and means for conveyingsaid expanded gas from the expansion means to the expanded gas channelof the exchanger.

2. A low temperature compressed gas refrigeration system comprising aheat exchanger containing a compressed gas channel for conducting acompressed gas through the exchanger and an expanded gas channelthermally bonded to the compressed gas channel for conducting anexpanded gas through the exchanger, a gas purifier communicating withthe compressed gas channel, a refrigeration load, means for conveyingsaid compressed gas after its passage through the purifier to the load,means for expanding said compressed gas to the lowest temperature pointin the system, means for conveying said compressed gas. after itspassage through the load to the expansion means, and means for conveyingsaid expanded gas from the expansion means to the expanded gas channelof the exchanger.

3. A low temperature compressed gas refrigeration system comprising acompressor, a cooler adapted to remove the heat of compression from acompressed gas, means for conveying the compressed gas from thecompressor to the cooler, av heat exchanger containing a compressedgaschannel for-conducting a compressed, gas through the exch nger; and,v anexpanded, gas channel thermally bonded to the compressed gas channel forconductinganexpandedgas.through the heat exchanger,

means for conveying the compressed gas from the cooler to the compressedgas channel of the exchanger, a gas purifier, means for conveying saidcompressed gas after its passage through the exchanger to the purifier,a refrigeration load, means for conveying said compressed gas after itspassage through the purifier to the load, an expansion engine forexpanding said compressed gas with the performance of external Work tothe lowest temperature point in the system, means for conveying saidcompressed gas after its passage through the load to the expansionengine, means for conveying said expanded gas from the expansion engineto the end of the expanded gas channel of the exchanger corresponding tothe outlet end of the compressed gas channel, and means for conveyingsaid expanded gas from the end of the expanded gas channel of theexchanger corresponding to the inlet end of the compressed gas channelto the compressor.

4. A method of producing low temperature refrigeration comprisingcooling a compressed gas to refrigeration temperature by passing it inout-of-contact heat exchange relation with a stream of the same gasafter expansion, passing the cooled compressed gas through a purifier,passing the cooled compressed gas from the purifier through arefrigeration load, expanding the compressed gas after its passagethrough the load to a temperature below refrigeration temperature, andpassing the expanded gas in out-of-contact heat exchange relation withthe compressed gas.

5. A method of producing low temperature refrigeration comprisingcompressing a gas, removing the heat of compression from the compressedgas, cooling the compressed gas to refrigeration temperature by passingit in countercurrent out-of-contact heat exchange relation with i astream of the same gas after expansion, passing the cooled compressedgas through a purifier, passing the cooled compressed gas from thepurifier through a refrigeration load, expanding the compressed gas withthe performance of external Work after its passage through the load to atemperature below refrigeration temperature, and passing the expandedgas in countercurrent outof-contact heat exchange relation with thecompressed gas.

6. A low temperature compressed gas refrigeration system comprising aheat exchanger having a compressed gas channel and an expanded gaschannel in heat-exchange relation, a purifier, a refrigeration load,expansion means, means for conducting a compressed gas through thecompressed gas channel to be cooled to a temperature only slightlyhigher than that attained in the expansion means, means for conductingthe gas from said compressed gas channel successively through thepurifier, the load and the expansion means to the lowest temperaturepoint in the system, and means for conveying the expanded gas from theexpansion means to the expanded gas channel of the heat exchanger.

References Cited in the file of this patent UNITED STATES PATENTS1,027,863 Von Linde May 28, 1912 1,264,807 Iefieries v Apr. 30, 19182,304,413 Kleucker Dec. 8, 1942 2,430,692 Touberg Nov. 11, 19472,548,335 Balogh Apr. 10, 1951

